This relates generally to imaging devices, and more particularly, image sensors having optically black pixels for backside illumination image sensors.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. Conventional image sensors are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology.
An image sensor may include an active pixel array having photodiodes. The image sensor may have optically black pixels that provide reference signals for use when processing signals from active pixels in the active pixel array. It is important that optically black pixels are optically isolated so as to receive as little light as possible.
Backside illumination (BSI) image sensors have been developed in which incoming light enters from a back side of a substrate. A dielectric stack, which is beneficial for isolating control and bias metal lines, is provided on a front side of a substrate. In BSI image sensors, incoming light does not pass through such a dielectric stack before reaching photodiodes in the substrate, which can result in improved pixel signals.
However, it may be difficult to provide good optical isolation for optically black pixels in BSI image sensors. Due to the position of the dielectric stack, stray light is sometimes reflected into the optically black pixels.
It would therefore be desirable to be able to provide image sensors having improved optical isolation for optically black pixels in backside illumination image sensors.